Process for substantial removal of phosphates from wastewaters

ABSTRACT

The invention is directed to a process for removing substantial amounts of soluble phosphates from wastewaters at primary treatment stages by means of chemical precipitation utilizing calcium chloride at a pH of 8-9.5, or a combination of calcium chloride and calcium hydroxide. An equivalent amount of calcia may be used in place of the calcium hydroxide.

United-States Patent [191 Lincoln et al.

[ 1 Feb. 13, 1973 PROCESS FOR SUBSTANTIAL REMOVAL OF PHOSPHATES FROMWASTEWATERS [75] Inventors: Patrick A. Lincoln, St. Louis; Philip G.Delamater, Alma, both of Mich.

[73] Assignee: Michigan Chemical Corporation, St.

Louis, Mich.

[22] Filed: May 10, 1971 [21] Appl. No.1 142,032

[52] US. Cl ..210/52 [51] Int. Cl ..C02c 1/40 [58] Field of Search..210/3-10, 52, 53,

[56] References Cited UNITED STATES PATENTS 3,431,200 3/1969 Davis et al..210/10 3,462,360 8/1969 McKinney ..210/14 X 3,440,165 4/1969 Davis eta] ..210/45 X 3,385,785 5/1968 Forrest et al. ...210/l 8 X 3,617,569 '11/197] Daniels et al ....2l0/53 X 3,423,309 1/1969 Albertson ..210/53 XPrimary Examiner-Michael Rogers Att0rneyJames J. Mullen 5 ABSTRACT 3Claims, No Drawings PROCESS FOR SUBSTANTIAL REMOVAL OF PHOSPHATES FROMWASTEWATERS The present invention relates to treatment and clarificationof raw wastewaters and sewage effluents. More specifically, theinvention is concerned with removal of soluble phosphates by thesubstantial precipitation thereof from wastewaters utilizing calciumchloride or a combination of calcium chloride and calcium hydroxide.

Recently, much concern has been shown over the quality of water found inour streams and lakes. It seemed that the extent and the degree of waterpollution were accelerating at uncontrollable rates, which have, in manyinstances caused various governmental agencies to institute stringentmeasures and requirements regarding wastewaters and sewage effluents.

Treatment of wastewaters and sewage effluents before discharging thesame into a river, lake or sea is a relatively new practice. In fact themajority of industrial wastewaters have gone untreated. Thus with theadvent of industrial growth and population increase the in-take ofpollutants by the various bodies of water could not be off-set bynatural dissemination. Consequently, the nutriently rich flow ofwastewaters has given rise to the growth of algae which on decompositiondeplete oxygen essential to marine and fish life. This process ofsuccessive enrichment and suffocation of a lake is called eutrophicationwhich is believed to threaten'many bodies of water. I

The nutrients found in wastewaters and sewage effluents comprise avariety of chemical compounds notable of which are phosphates andnitrates. Uses of phosphates and phosphorus-containing compounds inconsumer and agricultural products have been one of the maincontributors to such pollution. Means for removing these nutrientcompounds are therefore essential from the ecological point of view. Asto phosphates, most governmental standards require only 80% removal.

In most water-treatment plants removal of phosphates can be incorporatedas a step in the over-all process. In fact that is usually done wherevera secondary treatment is involved. This is in contrast to the processesinvolving primary treatment which utilizes sedimentation of thewastewater or its passage through some filtering means to remove mostsuspended solids. As can be seen, primary treatments cannot remove anysoluble materials, particularly phosphates. Secondary treatment involvesfurther treatment of the effluent from the primary treatment attemptingto reduce the dissolved solids by means of precipitating, coagulatingand/or flocculating agents followed by trickling filters, digestors orthe like. Because of their efficiency and low cost, digestors utilizingthe activated-sludge method has been widely adopted by manywater-treatment facilities since this method takes advantage of thebacteria and microorganisms found in wastewatersto collect or digest thedissolved and colloidal organic matter. On settling, the sludge iscollected and may be treated and used as fertilizer or otherwisedisposed of, such as by burying or land-filling. Use of theactivatedsludge, however, requires the proper maintenance and control ofseveral parameters and conditions such as the pH and BOD. BOD is a termreferring to biological oxygendemand which is a value indicating theamount of oxygen required in the biological decomposition of the organicmatter present. Thus it is important to keep the BOD value as low aspossible in the treated effluent.

The present invention contemplates a water-treatment system whichutilizes a chemical process to precipitate phosphates, especially inconnection with a digestor-activated sludge treatment step, although itmay find use in connection with other treatments. In one of itsprincipal aspects, the invention is directed to a chemical process forremoving the bulk of dissolved phosphates in wastewaters. In the priorart a variety of precipitating agents have been used such as lime, alum,ferric chloride, and both ferrous and ferric sulfates. The iron saltsare particularly useful because of their flocculent characteristicsexcept for their excessive cost which burdens financially the over-alltreatment. Lime has been used extensively to precipitate phosphates asit affords economic advantages not available with the others.Unfortunately lime has been shown to affect adversely thedigestonactivated sludge process because it makes the effluent from theprimary treatment stage too alkaline. It is well known that mostbacterial forms cannot survive long in media having pH values in excessof 9.5-l0.0. Thus the advantage provided by the action of themicro-organisms in the activated-sludge is impaired substantially.Considering the fact that the effluent from the primary treatment isexpected to flow on a continuous basis, any impaired stage thereafterpresents very serious problems such as stoppage or disposing theeffluents improperly. It is for this reason that many water-treatmentplants dislike the use of lime as means for precipitating phosphates.Lime or calcium hydroxide is a very strong base making pH adjustmentvery difficult particularly when it is realized that it operates best atpH of 9.5.

Thus to use an inexpensive precipitating agent such as calcium hydroxidewill require additional expenses in equipment and personnel to monitorthe alkalinity of the influent stream from the primary treatment intothe digestor tank and consequently the economic advantage is lost. Inthis connection it should be appreciated that water-treatment plantsutilizing secondary stages other than activated-sludge cannot tolerateeffluents having pH values in excess of 9.5. The reason for this isregulatory in that water disposed in lakes or rivers should be close toneutral, i.e., neither too alkaline nor too acidic.

Accordingly the main object of the present invention is to provide anagent for precipitating phosphates which is inexpensive and can beincorporated in present processes without additional cost ormodification. It has been found that dissolved phosphates in wastewatersand sewage effluents can be substantially removed by precipitating thesame with calcium chloride at a pH range of 8-9.5. Since mostwastewaters have pI-Is near neutral, any suitable or available base canbe used to provide the desired pH range of 8 I to 9.5. But because ofemphasis on cost and advantages consists of several forms, each of whichhas its separate chemical properties. These consist of the particulateorganic phosphates tied up in organic matter, orthophosphates andpolyphosphates. Generally wastewaters compromise equal parts of theortho and polyphosphates with the organic phosphates representing asmall fraction of the total. The bulk of the poly phosphates is amixture of pyrophosphates and tripolyphosphates with a minor amount ofmedium to long-chain polyphosphates.

Polyphosphates are hydrolyzed to the ortho form by enzymatic action andoften are in that form by the time they reach the treatment stage.Calcium ions furnished by either CaC1 and/r Ca(0H) -CaC1 combinationreact rapidly with the orthophosphates to precipitate a crystallinematerial believed to be a mixed salt of tricalcium phosphates and may berepresented by the formula Ca (0l-l)(P0 or Ca P0 -Ca (0H)(P0 in normalwastewaters and sewage effluents it has been found that 100 ppm of CaClat pH 8-9.5 will precipitate over 80 percent of the dissolvedphosphates. lf calcium hydroxide and calcium chloride are used incombination less calcium chloride is needed because of the Ca ionsfurnished by the lime. Of course adjustment of the CaCl dose may benecessary for exceptional or unusual phosphate concentrations.

The advantage provided by the use of calcium chloride is economicallydouble-edged. As a starting material the cost of CaCl is comparable toCa(OH) but more importantly the fear of lime overdosage, i.e.,

high pH values, is practically eliminated. Thus sophisticated pH controlequipment and/or personnel time to monitor them will no longer be amust. This is true even when part of the treatment is lime itself sinceit is not used to the extent necessary to precipitate all of thephosphates present. Moreover when lime is used alone as theprecipitating agent it lacks flocculating properties and the reactionwith phosphates does not affect the hydrogen ion concentration.Conversely, calcium chloride reaction is believed to release somehydrogen ions thereby decreasing thepH. Moreover, calcium chloride hasbeen found to possess flocculating characteristics resembling the ferricand ferrous salts in this respect. it can, then, be seen that the use ofcalcium chloride provides very unexpected results, i.e., some pH controland flocculent action both of which being necessary in'the effectiveremoval of soluble phosphates from wastewaters.

To further illustrate the present invention, the following examples areprovided showing some specific embodiments of the invention; they arenot, however, to be construed as in any way limiting the scope of theinvention.

EXAMPLE I A standard solution of orthophosphate was prepared so that itcontained 200 parts per million (ppm) phosphate calculated as PO Variouslower concentrations could be made to match actual'samples taken fromsewage effluents. Several sources of industrial and municipalwastewaters were examined for phosphate content and shown to containamounts ranging from 30 to 50 ppm. Of course, some wastewaters maycontain considerably higher contents of phosphates if they happen tocollect effluents from industrial plants utilizing for examplephosphoric acid as in pickling operations.

An aliquot of the standard solution given above was diluted to provide asolution containing 37 ppm of phosphate. The solution was made alkalineby using a suitable base such as sodium hydroxide. The pH was measuredto be about 8.l0 at which stage ppm of CaCl (in the form of anv aqueoussolution) was added to the phosphate solution. A white precipitate wasobserved soon after the addition. The precipitate was allowed to settleover a 24-hour period after which the pH of the supernatant solution wasmeasured at 7.10 and the phosphate was shown to have been removed to theextent of 87 percent.

When calcium chloride was added to a similar phosphate sample at pH 7.00no precipitate was observed. In fact after 24 hour-period the solutionwas slightly on the acid side as it had a pH of 6.70.

From the above it can be seen that calcium chloride is capable ofremoving, by precipitation, as high as 87 percent of the phosphatepresent in solution so long as the addition was made in alkaline media.

Higher amounts of phosphates are, of course, precipitated with calciumchloride by making the solution more alkaline. In Table I there is shownthe extent of phosphate precipitation with respect to initial pH' andits decrease over 24 hour-period. Data presented in Example I are alsoincluded for comparison.

TABLE 1 CaCl, added (ppm) 100 100 100 100 Initial pH 7.0 8.1 9.0 10.]Initial PO{"(ppm) 37.0 37.0 37.0 37.0 Final P0,,(ppm) 37.0 4.8 3.6 0.4Precipitation 0 87 93 99 pH after 24 hrs. 6.7 7.1 7.9 8.4

It is obvious that the composition of raw wastewaters would fluctuateand vary from day to day. By and large the pH of the raw wastewatersprior to treatment is close to neutral but on the alkaline side. Thesewaters, however, do contain considerable amounts of dissolved carbondioxide and bicarbonates which present difficulties in adjusting the pH.In this connection, it should be stated that aeration of wastewaterswould aid in removing some of the dissolved carbon dioxide and thuslower the amounts of the base needed to bring up the pH level. Additionof calcium hydroxide is generally done to neutralize acid components ofwastewaters and also to effect the precipitation of the solublephosphates. On a laboratory standard, Ca(Ol-l) is excellent at removingphosphates. On wastewaters, however, the matter is not as simple; pHvalues higher than 9.5, are quite harmful since at such levels mostactivated sludge operations would cease to function effectively'. It is,therefore, extremely important to control the pH. Often it is difficultto do so and lime does not appear very attractive.

EXAMPLE II To a neutral standard solution containing 37- ppm of PO; 100ppm of Ca(OH) was added (amounts provided herein are based oncalculations made with reference to standard solutions. Phosphate wasdeter mined by using stannous chloride method for orthophosphates asgiven in Standard Method for the Examination of Water and Wastewater,12th Edition, page 234. All additions are made via solutions in whichppm are related to volume, i.e., l cc contains so many ppm of theparticular reagent.) A white precipitate was observed to form veryreadily. After settling, the pH was shown to be 9.3. Phosphate contentin the supernatant solution is 7.0 ppm showing about 82 percentprecipitation. it appeared that additional lime was necessary toprecipitate the PO. completely. The added increments, however, follow analmost exponential rate. For example a lime addition of 135 ppmprecipitated hardly any additional phosphate but did increase the pH toabout 9.5. Complete precipitation of the phosphate was not effecteduntil the Ca(OH) exceeded 200 ppm and the pH level was over 10.5.wastewater effluents having a pH of over 9.5 would not be susceptible tofurther secondary treatment of activated sludge because most bacterialforms cannot survive in such an alkaline medium.

Utilizing the combination of calcium hydroxide and calcium chloride toprecipitate phosphates proved to be very effective. Calcium hydroxidewill increase the pH to acceptable levels and provide some concentrationof calcium ions whereas calcium chloride will help provide additionalcalcium ions to precipitate the substantial portion of solublephosphates. Thus to a standard solution containing 37 ppm of P0; wasadded 50 ppm Ca(OH) followed by 50 ppm of CaCl The pH was measured at8.5. Amounts of soluble P0 removed by precipitation were over 80percent.

Table 11 shows the analyses of raw sewage waters obtained from the townsof Alma and St. Louis, Mich. By comparison the analyses were typical inthat they fell within ranges observed in many towns and areas throughoutthe country. Fluctuation of values of the various components is alsotypical with occasional high or low values.

TABLE 11 Components of Sewage Effluents from Alma and St. Louis,Michigan PO pH Bicarbonate as free CO (pp g CaCOw g/ 39 7.5 30 7.6 287.7 260 31 53 7.6 245 41 Alma 38 7.5 34 7.5 232 18 3O 34 7.8 440 70 277.7 400 46 St. Louis 38 7.8 380 77 Table 111 is intended to show theextent of PO.{" removal by precipitation utilizing Ca(OH and acombination of Ca(OH) -CaCl on sewage effluents. Of note are the ratesof change of pH values and phosphate removal relative to time. Duringthese runs no attempt was made to exclude air. Decreases in pH valueswere observed and believed to be caused by carbon dioxide. Of course,formation of calcium carbonate can be expected particularly in the caseof lime addition when the pH is above 9.5. It is believed that someinhibition of carbonate formation is effected at lower pH values.

TABLE 111 Phosphate Precipitation Relative to Time and pH A. Ca(OHh 100ppm Time pH POP" POf precipitated (hr) (ppm) (711) 0.00 7.5 25 .0 0 0.259.5 6.0 76 0.50 9.5 5.5 78

200 ppm 100 ppm D. CaCl 200 ppm"'" E. CaCl 50 ppm Wastewaters used inthis Table were obtained from the sewage effluents of the city of Alma,Michigan.

* Any suitable base can be used to effect the desired pH values. Thushydroxides such as sodium, potassium, calcium and ammonium hydroxide canbe used.

It should be evident from the foregoing specification and examples thatthe term wastewater" is employed herein to indicate any raw wastewateror sewage effluent. In fact, the process of the present invention hasfound an exceptional utility in treating industrial wastewater which isrelatively high in phosphate content. Additionally, experience has shownthat the amounts of CaC1 needed to precipitate the phosphates. are notconsidered high with respect to chloride ions pollution. The calciumchloride can be added in solid form or as an aqueous solution. It ispreferable that a solution containing 15 to 45 percent by weight ofcalcium chloride is used since it provides a built-in safety for error,i.e., easier means of addition. Brines containing calcium chloride havebeen found to be adequate.

What is claimed is:

l. A process for substantially removing dissolved phosphates fromwastewater consisting essentially of the steps of adding calciumhydroxide to said water to provide an alkaline waste water having a pHrange of which process comprises the steps of adding a calcium hydroxidecontaining calcium chloride to said wastewater and adjusting the pHthereof to be in the range of 8 to 9.5 thereby precipitating the majorpart of said dissolved phosphates for consequent removal.

1. A process for substantially removing dissolved phosphates fromwastewater consisting essentially of the steps of adding calciumhydroxide to said water to provide an alkaline waste water having a pHrange of from 8 to 9.5, treating said alkaline water with calciumchloride to thereby precipitate and consequently remove the insolublephosphates formed.
 2. A process according to claim 1 wherein calciumhydroxide is added in amounts up to about 50 ppm.